Rechargeable batteries having high capacity when they are charged and discharged at a high electricity current and that remain stable when the charge and discharge process is repeated during a long period of time are in high demand. Such batteries, which include lithium secondary batteries, are used for example in electric vehicles, hybrid cars and the like.
Typically, two categories of lithium secondary batteries are known. A first category in which the negative-electrode is formed by using a material capable of absorbing and discharging lithium ions, and a second category in which the negative-electrode is formed by using metallic lithium. Lithium secondary batteries in the first category present at least some advantages over those in the second category. For example in the first category, safety of the battery is enhanced since there is less dendrite deposit and thus a short circuit between the electrodes is less likely to occur. Also, batteries in the second category generally have higher capacity and energy density.
In recent years, lithium secondary batteries in which a negative-electrode is formed by using a material capable of absorbing and discharging lithium ions have been in high demand. Extensive research has been conducted aiming at improving the capacity of the battery when it is charged and discharged at a high electric current and also at improving its performance and life cycle, for up to several tens of thousands cycles. It has been found that the capacity of the battery is improved by decreasing its electrical resistance. Also, the following have been found to be advantageous: (a) use of a positive-electrode material comprising a lithium metal oxide as the reactive substance and a negative-electrode material comprising carbon, leads to a high capacity battery; (b) the total reactive surface of a substance in the battery is increased by decreasing the mean size diameter of the particles of the substance, or the reactive surface of the electrode is increased by optimizing the design of the battery; (c) liquid diffusion resistance is decreased by making a separator thin.
When the mean size diameter of the particles of the reactive material is small, the total reactive surface is increased. However, this necessitates an increase of the amount of binder used in the material. As a result, it becomes quite challenging to obtain a battery that has a high capacity. In addition, the positive-electrode and negative-electrode materials have a tendency to peel or drop from the metal foil on which they are deposited. And since these materials are electricity collectors, an internal short circuit inside the battery is likely to occur, resulting in a decrease in the voltage of the battery and thermal runaway. Safety of the lithium secondary battery is thus impaired.
Research has been conducted aiming at designing methods for increasing the adherence of the positive-electrode and negative-electrode materials to the metal foil on which they are deposited. Such methods include for example altering the type of binder, as disclosed for example in Japanese laid-open patent application No. 5-226004.
Also, methods have been designed for allowing the lithium secondary battery to have a high capacity when it is charged and discharged at a high electric current. For example, use of conductive carbon material to decrease the electrical resistance of the electrode has been disclosed; see for example Japanese laid-open patent applications No. 2005-19399, No. 2001-126733 and No. 2003-168429.
Although a suitable choice of binder used in the reactive material allows for an increase of the capacity of the battery, this does not appear to have a positive effect on the improvement of the property of the battery whereby it has a high capacity when charged and discharged at a high electric current, even when the electrical resistance of the electrode is decreased.
When the battery is cyclically charged and discharged at a high electric current, the positive-electrode and negative-electrode materials tend to expand and contract. This leads to conductive paths of particles between the positive and negative electrodes being impaired. As a result, a high electric current cannot be circulated early on when the battery is used, and the battery thus has a short life span.
In recent years, lithium metal phosphate compounds such as olivine-type lithium iron phosphate as reactive substances of the positive-electrode in lithium secondary batteries have attracted attention; see for example Japanese laid-open patent applications No. 2000-509193 and No. 9-134724. Indeed, this reactive substance is safe and contributes to a decrease in the cost of the battery since it is inexpensive. However, the substance has a high electrical resistance and attempts to decrease the resistance have been found quite challenging.